In a park lock mechanism of the type set forth above according to the prior art, the pawl means on a rotatable lock arm is composed generally of a single locking pawl and a particular gear other than power transmission gears is generally employed as the gear for the park lock mechanism. Such particular gear is employed for the purpose of eliminating the necessity of providing a large rigidity for an operating system of the lock arm. That is, the single locking pawl is projected into a tooth space of the gear and prevents a forward directional rotation of the gear by an engagement of its one side surface with one of the tooth surfaces of the gear at one side of the tooth space and a backward directional rotation of the gear by an engagement of its the other side surface with the other tooth surface at the other side of the tooth space. A large force is applied to the one side surface of pawl by the one tooth surface of gear or to the other side surface of pawl by the other tooth surface of gear particularly when the vehicle is parked at a slope, so that a large torque or rotational moment is applied to the lock arm by the gear in a direction of causing a lock-releasing rotation of the arm. From this, the operating system of the lock arm must have a rigidity large enough to keep an engaged condition of the locking pawl with the gear. Because the torque or rotational moment applied to the lock arm by the gear is increased with the increase in pressure angle of the gear, a transmission gear which is designed to have a large pressure angle for heightening its strength will apply a large torque or rotational moment to the rotatable lock arm so that a particular gear of a small pressure angle is employed as the one for the park lock mechanism.
However, even when a gear of a small pressure angle is employed in a park lock mechanism, such torque or rotational moment is not necessarily small depending on the location of rotational axis of the lock arm and the like. Further, it is often true that different amounts of torque or rotational moment are caused in a vehicle between the two cases where the vehicle tending to run spontaneously into a forward direction is kept from starting and where the vehicle tending to run spontaneously into a backward direction is kept from starting. From this, a prior art mechanism is known, as shown in JP, U No. 56-21261, in which there is employed a particularly profiled gear having tooth surfaces of different pressure angles at the upstream side of forward directional rotation of the gear and at the upstream side of backward directional rotation of the gear so that torque or rotational moment applied to the operating system for the rotatable lock arm will be equalized at the two cases set forth above so as to lighten the burden to the operating system.
However, such gear having tooth surfaces of one and the other rotational sides of different pressure angles requires particularly shaped gear-cutting and gear-finishing tools so that it is very expensive to manufacture. The structure that gear of a small pressure angle other than transmission gears is employed in a park lock mechanism is not preferred in view of cost required for parts and assembling of a vehicle transmission and also in view of space required for the transmission.
Accordingly, a primary object of the present invention is to provide a novel park lock mechanism which permits the employment of a gear having a relatively large pressure angle, such as a transmission gear, without the need of reinforcement of the lock-actuating system.